[path] Fix root path mocking.

recipe_modules/path/api.py

 # Copyright 2013 The LUCI Authors. All rights reserved.
 # Use of this source code is governed under the Apache License, Version 2.0
 # that can be found in the LICENSE file.
 
 """All functions related to manipulating paths in recipes.
 
 Recipes handle paths a bit differently than python does. All path manipulation
 in recipes revolves around Path objects. These objects store a base path (always
 absolute), plus a list of components to join with it. New paths can be derived
 by calling the .join method with additional components.
@@ skipping 66 matching lines @@
   def _initialize(self):
     self._initialize = lambda: None
     for path in self._initial_paths:
       self.add(path)
     self._initial_paths = None
     self.contains = lambda path: path in self._paths
 
   def add(self, path):
     path = str(path)
     self._initialize()
-    while path:
+    prev_path = None
+    while path != prev_path:
       self._paths.add(path)
-      path = self._path_mod.dirname(path)
+      prev_path, path = path, self._path_mod.dirname(path)
 
   def copy(self, source, dest):
     source, dest = str(source), str(dest)
     self._initialize()
     to_add = set()
     for p in self._paths:
       if p.startswith(source):
         to_add.add(p.replace(source, dest))
     self._paths |= to_add
 
@@ skipping 331 matching lines @@
     if name in self.OK_ATTRS:
       return getattr(self._path_mod, name)
     if name in self.FILTER_METHODS:
       return string_filter(getattr(self._path_mod, name))
     raise AttributeError("'%s' object has no attribute '%s'" %
                          (self._path_mod, name))  # pragma: no cover
 
   def __dir__(self):  # pragma: no cover
     # Used for helping out show_me_the_modules.py
     return self.__dict__.keys() + list(self.OK_ATTRS + self.FILTER_METHODS)